1. Field of the Invention
The present invention relates to a method for reinforcing a slope, and more particularly to such a method, which is capable of recovering and restoring the slope as the status quo so as to maintain its stability without additional reduction of its gradient using a reverse analysis technique.
2. Description of the Prior Art
In the case of artificially constructing a slope by excavating or cutting a natural sloping land, the slope gradually loses its stability as time goes by and is finally degraded or deformed to do damage to a person""s life or property. Additional cutting or reinforcement, thus, is needed when there is a problem in the stability of the excavated or cut slope, but it is impossible in some cases to additionally cut the slope in view of its topographical features. The present invention provides a method for reinforcing the already-constructed slope so as to make it possible to stabilize it and restore it to its own natural state by means of an environmentally favorable method of construction.
A reinforcing method by a soil nailing method has been conventionally used as the method of reinforcing the slope. The conventional slope reinforcing method by the soil nailing method is based on a limit equilibrium analysis in which a static limit equilibrium theory is introduced to examine an overall failure surface over the entire soil. Such a soil nailing method includes Davis method proposed by Shen et al. in 1981, a method proposed by Gassier and Gudenhus and considering only tensile capacity of a reinforcement member (soil nail), and a French method, proposed in 1983, considering an effect of shear capacity on the overall stability and bending stiffness in accordance with the tensile capacity of the reinforcement member, the last one having been practically used up to the present. The soil nailing method is a method in which soil parameters of the ground are determined in advance on the basis of results from a laboratory test and a field test in situ, an internal stability condition is studied to be adapted to characteristics of the reinforcement member, and then an external stability condition is studied. Herein, the internal stability condition is a stability condition for the reinforcement member capable of resisting a slope failure force under a condition for limit equilibrium state, and the external stability condition is a stability condition for such a case that a slope failure line is located at an outer periphery of the reinforcement member. In the soil nailing method, the surface of the slope is subjected to a surface treatment by a stiff structure using concrete or shotcrete. At present, this structure constructed by the soil nailing method is practically used as a vertical excavation-type bracing structure.
FIG. 1 is a schematic diagram showing the slope reinforcing method in accordance with the soil nailing method. The soil nailing method comprises the steps of studying a underground water level and special conditions in connection with an applicable limit; determining soil parameters by a field in situ test, a borehole pressure meter test, a laboratory soil test, etc.; calculating a skin friction resistance by a pull-out test to determine an adhesion force of a nail; determining construction spacing, drilling angles and lengths of the nails on the basis of the determined soil parameters and adhesion force to study an internal stability condition; calculating a post-reinforcement stability by iterative calculations on assumed slope failure line of the ground; planning design of a construction section in accordance with the determined results and constructing the nails; and treating the constructed surface with a stiff structure of concrete or shotcrete.
The slope reinforcing method by the soil, nailing method, however, has no backup measures to counter a case that the values of the soil parameters (a cohesion (C), an internal friction angle (xcfx86), a construction density (xcex3), an elastic modulus (Es), a limiting pressure (p1) or the like) applied to the design do not correspond with field deformation behavior, and thus cannot overcome problems arising due to deciding the soil parameters determined by the field test in situ, the laboratory test and so forth as representative values. Also, the method cannot predict maximum tensile and shear forces formed within the given reinforcement member in a certain position, but provides only an overall factor of safety. That is, the following expression is established:                               V          t                =                                                            R                c                                                              [                                      1                    +                                          4                      ⁢                                              xe2x80x83                                            ⁢                                                                        tan                          2                                                ⁡                                                  (                                                                                    π                              2                                                        -                            α                                                    )                                                                                                      ]                                                  1                  2                                                      ≅                          T              t                                =                      4            ⁢                          xe2x80x83                        ⁢                          V              t                        ⁢                          tan              ⁡                              (                                                      π                    2                                    -                  α                                )                                                                        [Exp.  1]            
wherein Vt is a shear force, Tt is a tensile force, Rc is a shear strength, and xcex1 is an angle of a potential failure plane. As seen from Expression 1, only the tensile force acts if xcex1=0 and only the shear force is effective if   α  =      π    2  
because there is a relationship of       R    c    =                    R        n            2        .  
The Davis method and French method are typically cited as basic analysis techniques of slope reinforcement by the soil nailing method. The Davis method considers only a tensile resistance and the French method considers a tensile resistance together with the shear resistance (cf. Technical Teaching report 78, Earth Reinforcement, 1989. 12, The Korean Highway Corporation).
According to the analysis by the French method, the tensile force within the upper reinforcement member must be 0 when an estimated potential failure line actually has a longitudinal extension direction   (      α    =          π      2        )
in an upper portion of the slope, but the tensile force is practically strengthened in the reinforcement member, thereby causing a problem in analysis.
As stated above, the conventional reinforcing method by the soil nailing method is a method in which an overall surface treatment of a nail head with concrete or shotcrete is performed as the final process after the soil nail reinforcement, thus having many problems, for example, spoilage of a fine view, difficulty in maintenance, lack of environmental intimacy due to spoiling of a natural scene and the like. Besides, since the analytic technique is one in which a field investigation, sampling, a laboratory test, a field location test (PMT), etc. are performed in advance to analyze ground strength characteristics and then the analyses of the slope stability and the reinforcing method are conducted on the basis of results of the ground strength characteristics, it not only requires a heavy cost and a long time, but often causes a problem in that the theoretical strength characteristics do not correspond with the actual field conditions. That is, there is a problem in that a failure model about a theoretical analysis does not correspond with a field failure model.
A countermeasure to reinforce a slope requires a rapid, accurate and safe reinforcing method capable of minimizing damage to a person""s life and property.
The present invention relates to such a method, in which a slope stability analysis is performed while ground strength characteristics suitable to a field failure model are most rapidly and easily analyzed by applying a reverse analysis technique based on field ground deformation characteristics so as to be make it possible to rapidly judge the-above mentioned problems at a low cost, and then a reinforcement construction is rapidly and safely carried out.
For the purpose of this, the present invention provides an environmentally favorable method of slope earth reinforcement without spoilage of a natural environment, which comprises a process of reversely analyzing the field ground deformation characteristics of the unstable slope to make it possible to judge the ground strength characteristics and a process of recovering and restoring the unstable slope by introducing and applying an earth reinforcement theory, i.e., a theory that an apparent cohesion is increased by reinforcement members so as to make it possible to secure stability.
That is, the present invention has been made to solve the above-mentioned problems and to prevent a slope from gradually losing its stability as time goes by and being finally degraded or deformed to do damage to a person""s life or property, it is an object of the present invention to provide a reinforcing method for environmentally favorably, economically and rapidly reinforcing such an unstable slope without removal thereof, which comprises a process of accurately and rapidly determining ground strength characteristics of the deformed slope by applying a reverse analysis technique so as to make it possible to most economically and rapidly reinforce the unstable slope, a process of providing slope drain holes (subterranean horizontal drain holes) in the slope in order to suppress action of pore water pressure, using a reinforcing steel bar as a reinforcement member, filling grout composed of cement, water and high fluidizing agent around the reinforcing steel bars to integrate the reinforcement members with ambient earth and rock and so to form reinforced earth with permeation and cementation of the grout in micro-cracks existing within the unstable slope, thereby making it possible to most rapidly and safely reinforce the slope applying an earth reinforcement theory, i.e., a theory that an apparent cohesion is increased by the reinforcement members, and a process of treating a surface portion of the slope by covering artificial greening soil covering containing natural monofilaments so as to make vegetation growth on the slope possible, thereby environmentally favorably reinforcing the slope without spoilage of natural environment.
To accomplish this object, there is provided a method for reinforcing a slope in accordance with the present invention, the method comprising the steps:
studying a underground water level, slope configuration, a soil condition status and rock joint orientation in connection with an applicable limit of the slope, on the basis of which soil parameters, including a cohesion and an internal friction angle, are determined using the Janbu method so as to be adapted to characteristics of the deformed ground;
analyzing stability of the slope using the soil parameters determined by the Janbu method to estimate a driving force and a resistance force of the slope;
planning a construction section of a reinforcement zone to be constructed with reinforcement members in order to increase the resistance force of the slope;
determining a position and a quantity of subterranean horizontal drain holes in consideration of the underground water level condition to study an external stability;
checking an internal stability within the reinforcement zone against a critical failure section in consideration of a pull-out force and a shear capacity of the reinforcement member; and
preparing design drawings so as to satisfy the external and internal stabilities and carrying out a reinforcement construction work.
An apparent cohesion increasing with construction spacing between the reinforcement members is preferably       C    xe2x80x2    =            3.6              γ        _              ∼          4.2              γ        _            
when a SD40:xcfx8625M/M reinforcing steel bar is used,       C    xe2x80x2    =            4.9              γ        _              ∼          5.6              γ        _            
when a SD40:xcfx8629M/M reinforcing steel bar is used,       C    xe2x80x2    =            5.9              γ        _              ∼          7.0              γ        _            
(t/m2) when a SD40:xcfx8632M/M reinforcing steel bar is used as a nail bar.
Preferably, the step of carrying out the reinforcement construction work comprises the steps of: insert-laying the reinforcement members in the slope in accordance with the design drawings; mixing cement, water and high fluidizing agent with each other to produce grout and gravitationally injecting the grout around the reinforcement members; laying slope drain holes in the slope in such a manner that they extend beyond the reinforcement zone in accordance with the design drawings; installing main earth-pressing steel plates, PVC-coated wire mesh and sub earth-pressing steel plates to fix the reinforcement members; and treating surfaces of the slope with general artificial greening soil covering or artificial greening soil covering mixed with natural monofilaments by a spray attaching vegetation method.
It is preferred that a safety factor of the slope is 1.4 or more in the construction section of the reinforcement zone.
As for a weathered residual soil layer slope or a rock mass slope having remarkable joint orientation, the step of an determining the soil parameters may be performed by determining a dip angle (a bedding plane angle or a plunge angle) (xcex8) of the slope joint as the internal friction angle (xcfx86) and inversely calculating a cohesion (C) at the determined internal friction angle under a condition for limit equilibrium state Fsxe2x89xa61.0.
As for an unsaturated earth cut slope ground, the step of determining the soil parameters may be performed by determining the internal friction angle (xcfx86) through a direct shear test and inversely calculating the cohesion (C) at the constant internal friction angle (xcfx86=const.) under a condition for limit equilibrium state Fs=1.0.
In the case of degradation or deformation of the slope, the step of determining the soil parameters may be performed by determining the internal friction angle (xcfx86) through the direct shear test and inversely calculating the cohesion (C), considering an estimated failure line under a condition for limit equilibrium state of 0.85xe2x89xa6Fsxe2x89xa61.03.
In the case that the slope is unstable and forms an irregular stratified profile corresponding to a limit equilibrium state, the step of determining the soil parameters may be performed preliminarily by assuming that a critical failure line passes through the lowest portion of an upper stratum of the slope, determining the internal friction angle (xcfx86r) through the direct shear test for a specimen of the upper stratum of the slope and inversely calculating the cohesion (C) under a condition for limit equilibrium state 0.9xe2x89xa6Fsxe2x89xa61.05, and secondarily by assuming that the critical failure line passes through the lowest portion of a lower stratum of the slope, determining the internal friction angle (xcfx86rxe2x80x2) through the direct shear test for a specimen of the lower stratum of the slope and inversely calculating the cohesion (Cxe2x80x2) under a condition for limit equilibrium state 0.9xe2x89xa6Fsxe2x89xa61.05.